ES6 symbols: turn symbols into a proper primitive type

src/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 195 matching lines @@
   V(global_string, "global")                                             \
   V(ignore_case_string, "ignoreCase")                                    \
   V(multiline_string, "multiline")                                       \
   V(input_string, "input")                                               \
   V(index_string, "index")                                               \
   V(last_index_string, "lastIndex")                                      \
   V(object_string, "object")                                             \
   V(prototype_string, "prototype")                                       \
   V(string_string, "string")                                             \
   V(String_string, "String")                                             \
+  V(symbol_string, "symbol")                                             \
+  V(Symbol_string, "Symbol")                                             \
   V(Date_string, "Date")                                                 \
   V(this_string, "this")                                                 \
   V(to_string_string, "toString")                                        \
   V(char_at_string, "CharAt")                                            \
   V(undefined_string, "undefined")                                       \
   V(value_of_string, "valueOf")                                          \
   V(stack_string, "stack")                                               \
   V(InitializeVarGlobal_string, "InitializeVarGlobal")                   \
   V(InitializeConstGlobal_string, "InitializeConstGlobal")               \
   V(KeyedLoadElementMonomorphic_string,                                  \
@@ skipping 2801 matching lines @@
   AssertNoAllocation no_alloc;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_